This invention relates to an electronic component test socket and a process for making such a socket.
Changes to the integration density of electronic components have been accompanied by an increase in the number of their input and output terminals. Furthermore, the continuously increasing values of the operating frequency of circuits have reduced the size of component interconnections and packages.
A series of tests needs to be carried out on electronic circuits at the time of manufacturing, and particularly after the end of manufacturing, to check that they are working correctly. These tests are made by connecting input and output terminals to test equipment designed to automatically carry out a number of measurements.
Since electronic circuits have to be disconnected from test equipment after they have been tested, test sockets are used that hold the circuits to be tested and facilitate the temporary electrical connection of input and output terminals of electronic circuits with the test equipment.
The invention is used for applications to test all types of circuits, either in the form of unfinished circuits on a semi-conductor wafer, or packaged. In particular, the invention may be used for testing flip-chip type circuits (with balls made of a meltable material), or CSP (Chip Scale Package) or BGA (Ball Grid Array) type circuits.
Document (1), for which the reference is given at the end of the description, describes a test socket made from a rigid silicon board. The board comprises etched cavities, the walls of which are lined with a metallic layer. The metallic layer is used as a test area and is connected to an outside test equipment.
This type of socket is particularly suitable for testing circuits for which the input and output terminals are lined with balls made of a meltable material. These balls then come directly into contact with the test areas of the socket cavities.
The use of a socket for which the pitch of the cavities is adapted to suit the pitch of the balls made of a meltable material on one face of a component, makes it possible to simultaneously carry out tests on a large number of inputs and outputs. However, this type of socket has a number of limitations.
It is possible that the face of the component on which the input and output terminals are located may have a planeness defect. Furthermore, the diameters of the balls made of a meltable material on the terminals may not be perfectly uniform. Since there is no way of compensating for these inequalities, the contact between some balls and some test areas could be imperfect or uncertain. Therefore, this type of test socket cannot be used for an entire wafer of integrated circuits, or for a large package.
Another disadvantage is related to the complex and expensive use of silicon for making sockets.
Finally, since the balls that are usually spherical, come into contact with the approximately plane surfaces of cavities, the electrical contact is limited to a small area and is not always guaranteed to be optimum.
Document (2), which is also referenced at the end of this description, describes improvements for making contact pads for test sockets. These improvements are designed to enable adaptation of the contact pads to different diameters and heights of the balls made of a meltable material.
Without going into details of the various embodiments mentioned, it is found that the sockets are not entirely free of planeness constraints of the wafers in the circuits to be tested, or at least their manufacturing cost is relatively high. Some contact pads also have limitations related to their large size compared with the pitch of the input and output terminals of the circuits to be tested, or limitations related to the relatively low conductivity of some materials used to make them.
One purpose of this invention is to propose a test socket for components that does not have any of the limitations of sockets known according to the state of the art presented above.
In particular, one purpose is to propose a test socket that can be used for components provided with several input-output terminals on one contact face, but for which this face may have a planeness defect.
Another purpose is to propose such a socket that guarantees excellent contact quality with circuit input-output terminals despite a disparity of the values of the diameter of contact balls, if any, fitted on input-output terminals.
Another purpose of the invention is to propose a socket adapted to checking highly integrated electronic circuits with a high density of input-output terminals.
Another purpose of the invention is to propose a simple and economic process for production of such a test socket.
More precisely, in order to achieve these purposes, a test socket is proposed for an electronic component comprising a embossed support layer comprising several embossments with projecting relief, the embossments being provided, near the top of the embossment, with at least one conducting test area that may be brought into electrical contact with a terminal of the component.
The embossments may advantageously be designed so that they can deform independently of each other.
The embossed structure of the support layer, and the fact that the test areas are arranged at the top of the embossments, means that a very large number of test areas can be placed on a small area. It also makes it possible to separate a higher xe2x80x9clevelxe2x80x9d containing test areas from a lower xe2x80x9clevelxe2x80x9d for example reserved for a set of interconnection tracks connecting the test areas to a test circuit.
Preferably, each embossment comprises a single test area and is associated with a single input and output terminal for a circuit type to be tested. The distribution of the embossments and their spacing are dictated by the distribution of the input and output terminals for the type of circuit for which the test socket is to be used. For example, the embossments may be distributed in rows and columns, in a regular network.
According to one particular aspect of the invention, the support layer may be made essentially from a flexible material. A flexible material means a material that is sufficiently flexible and elastic so that it can be deformed as much as necessary to compensate for any planeness defects or inequalities in an electronic circuit, an electronic circuit package to be tested or connection terminals of such a package or circuit. A very slight pressure applied to the component to be tested locally deforms the support layer, guaranteeing good contact between all terminals and the corresponding test areas. In particular, deformation compensates for inequalities in the height and/or diameter of contact balls fitted on component input and/or output terminals.
According to another particular aspect of the invention, the socket may comprise peripheral contact areas connected to the test areas through conducting tracks extending essentially between the embossments. Therefore the conducting tracks are essentially located outside the plane of the test strips, so that this plane is available to enable greater integration of the test areas.
At the top of the embossments, the test areas may be plane or they may form a depression or they may be covered by upstands in the form of contact balls, for example made of a meltable material. The shape of the test areas is adapted to the shape or the type of coating on the input and output terminals of the circuits to be tested. When these terminals are already equipped with balls made of a meltable material, the test areas may be plane or they may form a depression with a shape complementary to the shape of the balls.
The flexible support layer may either be self-supporting or it may be added onto a rigid support. This support may accessorily comprise a test circuit for testing components.
The face of the support layer opposite the face equipped with test areas has a reentrant relief complementary to the projecting embossments. The recesses in this relief may possibly be filled in by an elastic material such as an elastomer.
The invention relates to a process for making the socket. This process comprises the following steps:
a) the function in a substrate of cavities forming relief with a shape corresponding to or complementary to an embossment of a support layer to be formed,
b) making a support layer consisting of a layer of flexible material on the substrate, so as to apply embossments corresponding to the relief of the substrate onto the layer, and separation of the support layer from the substrate,
c) the formation of test areas near the top of the embossments, by forming a metal layer.
Step a) to form cavities may be done either by etching or micromachining of a single or multi-layer substrate, or by electroforming of appropriate patterns.
Step b) to make the support layer may be done either by moulding, or by layer spreading.
The steps in the process may be done in the order described above. In this case, step c) includes the deposition of a metallic layer on the support layer, then etching of this layer by means of a mask fixing the size and location of the test areas.
Step c) may also be done before the support layer is separated from the substrate.
According to one variant, step c) may also be done before step b). In this case, a metal is deposited on the substrate used as a mould and is then etched before the support layer is formed.
The metallic layer may be of the single layer or multi-layer type.
A small size projecting relief can be formed on the substrate making up the mould, corresponding to the depressions at the top of the embossments. For example, small upstands may be formed at the bottom of the substrate cavities before step b). These upstands then define the depressions at the top of the embossments in the support layer when it is being moulded.
Other characteristics and advantages of the invention will become clear after reading the following description with reference to the figures in the attached drawings. This description is given purely for illustrative purposes and is in no way limitative.